GFR Calculator (eGFR)
Enter your serum creatinine, age, and sex to estimate your glomerular filtration rate using the 2021 CKD-EPI creatinine equation, the current standard recommended by the National Kidney Foundation and the American Society of Nephrology. You get your eGFR in mL/min/1.73 m², your KDIGO stage (G1 through G5), estimated kidney function as a percentage, and a plain-language interpretation. Switch creatinine units between mg/dL and micromol/L. Results update instantly as you type.
What is GFR and why does it matter?
Glomerular filtration rate (GFR) is the volume of blood your kidneys filter per minute per 1.73 square metres of body surface area. It is the single best overall index of kidney function. A GFR of around 100 mL/min/1.73 m² is typical for a healthy young adult; it declines by roughly 0.75-1 mL/min/year with normal ageing after age 40, and faster in people with chronic kidney disease (CKD). Because measuring true GFR requires infusing inulin or a radioactive tracer, clinicians instead estimate it (eGFR) from a routine blood test for serum creatinine plus age and sex. The current recommended approach is the 2021 CKD-EPI creatinine equation, which replaced the older MDRD and 2009 CKD-EPI equations because it is more accurate across the full GFR range and no longer includes a race variable. An eGFR below 60 mL/min/1.73 m² persisting for at least three months meets one of the key criteria for diagnosing CKD.
The 2021 CKD-EPI equation explained
The 2021 CKD-EPI creatinine equation uses four variables: serum creatinine (Scr, in mg/dL), age (years), and biological sex. The formula is: eGFR = 142 x min(Scr/K, 1)^alpha x max(Scr/K, 1)^-1.200 x 0.9938^Age x 1.012 [if female]. For females, K = 0.7 and alpha = -0.241; for males, K = 0.9 and alpha = -0.302. The min and max functions compare the creatinine-to-kappa ratio to 1, effectively splitting the calculation into a below-threshold and an above-threshold region, each with a different slope. The 0.9938^Age term captures the expected decline in GFR with ageing; the female multiplier (1.012) corrects for lower average creatinine production per unit of lean mass in women. Creatinine must be measured with an IDMS-standardized assay, which is standard in most modern clinical laboratories. This equation is validated for adults aged 18 and older; paediatric patients need the Schwartz or CKiD U25 equation instead.
Understanding KDIGO stages and what they mean clinically
The Kidney Disease: Improving Global Outcomes (KDIGO) 2012 guidelines classify GFR into six categories (G1 through G5, with G3 split into G3a and G3b) to guide clinical management. G1 (>=90) and G2 (60-89) represent normal or near-normal filtration; at these stages, CKD can still exist if there is proteinuria, haematuria, or structural kidney damage, so a urine albumin-to-creatinine ratio (ACR) test is essential. G3a (45-59) and G3b (30-44) mark the range where complications start to emerge, including anaemia from reduced erythropoietin production, bone mineral disease from impaired phosphate handling, and elevated cardiovascular risk. Medication doses need adjustment for many drugs at eGFR below 60. G4 (15-29) is advanced disease: preparations for renal replacement therapy - hemodialysis, peritoneal dialysis, or kidney transplantation - should begin even before symptoms appear. G5 (<15) is kidney failure, where renal replacement therapy is required to sustain life.
Limitations of creatinine-based eGFR
Serum creatinine is produced by muscles from creatine phosphate, so people with high muscle mass (bodybuilders, manual workers) have higher creatinine at the same true GFR, making their eGFR appear lower than it actually is. Conversely, older adults, people with muscle-wasting diseases, and those with very low body weight have low creatinine despite reduced kidney function, which can make their eGFR appear higher than the truth. Diet also plays a role: eating a large amount of cooked red meat shortly before a blood test transiently raises creatinine and lowers the calculated eGFR. For patients where these factors are significant, the cystatin C-based or combined creatinine-cystatin C CKD-EPI equations offer a more reliable estimate, because cystatin C production is not affected by muscle mass. A single eGFR reading is also a snapshot: the KDIGO definition of CKD requires that impaired function persist for at least three months, so repeat testing is essential before any diagnosis.
KDIGO GFR categories and clinical action thresholds
| Stage | eGFR (mL/min/1.73 m²) | Description | Key actions |
|---|---|---|---|
| G1 | >= 90 | Normal or High | Treat risk factors; annual monitoring if at risk |
| G2 | 60-89 | Mildly Decreased | Monitor every 12 months; urine ACR check |
| G3a | 45-59 | Mildly to Moderately Decreased | Monitor every 6 months; adjust medications |
| G3b | 30-44 | Moderately to Severely Decreased | Nephrology referral; monitor every 3-6 months |
| G4 | 15-29 | Severely Decreased | Prepare for renal replacement; monitor every 3 months |
| G5 | < 15 | Kidney Failure | Dialysis or transplant; specialist management |
KDIGO 2012 GFR categories. CKD is defined as eGFR < 60 mL/min/1.73 m² for >= 3 months, or eGFR >= 60 with markers of kidney damage (albuminuria, haematuria, structural abnormality).
Frequently asked questions
What is a normal eGFR?
For healthy young adults, a GFR of 90-120 mL/min/1.73 m² is typical. GFR naturally declines with age (roughly 0.75-1 mL/min/year after 40), so a result of 75 in a 70-year-old may be appropriate ageing rather than disease. KDIGO classifies eGFR at or above 90 as G1 (normal or high) and 60-89 as G2 (mildly decreased). A result below 60, if confirmed on two separate tests at least three months apart, meets one criterion for chronic kidney disease.
Does this calculator use race in the formula?
No. This calculator uses the 2021 CKD-EPI creatinine equation, which removed the race variable that appeared in the older 2009 version. The 2021 update was jointly recommended by the National Kidney Foundation and the American Society of Nephrology after a task force concluded that including race in medical algorithms can lead to inequitable care and that the equation performs well across racial and ethnic groups without it.
What is the difference between GFR and eGFR?
True GFR is measured by infusing a substance the kidneys freely filter but neither reabsorb nor secrete - classically inulin or a radio-labelled tracer - and measuring how quickly it clears from blood. This is precise but expensive and time-consuming. Estimated GFR (eGFR) uses a blood creatinine level plus age and sex in a validated equation to approximate true GFR. The CKD-EPI equation is the current standard for routine clinical use.
Can my eGFR fluctuate from day to day?
Yes. Serum creatinine and therefore eGFR can shift by 10-20% from day to day based on hydration, recent exercise, diet (especially red meat intake), certain medications such as trimethoprim or cimetidine, and timing of the blood draw. An acute illness or surgery can cause a sharp temporary drop. That is why a single low eGFR alone does not diagnose CKD: KDIGO requires the finding to persist for at least three months on at least two separate occasions.
What should I do if my eGFR is below 60?
An eGFR below 60 that persists for three months or more meets a key criterion for CKD and warrants discussion with your doctor. Useful next steps include: a urine albumin-to-creatinine ratio (ACR) to check for protein loss; blood pressure management to slow progression; review of all medications for dose adjustment or drugs to avoid (especially NSAIDs and certain antibiotics); blood tests for anaemia, electrolytes, and bone minerals; and referral to a nephrologist if eGFR is below 45 or declining rapidly.
Why does the formula give different results for male and female?
Women typically have less skeletal muscle than men of similar size, so they produce less creatinine per kilogram of body weight. A creatinine reading of, say, 0.9 mg/dL means something different in a woman (where it is above her average) than in a man (where it is close to average). The CKD-EPI equation uses sex-specific kappa and alpha constants, plus a small female multiplier (1.012), to account for these physiological differences and produce an accurate eGFR for each group.
How accurate is this calculator for very muscular or very lean people?
Not as accurate. Athletes or people with high muscle mass generate more creatinine per unit of true GFR, so their eGFR will be underestimated: they may read as G3a or G3b when their kidneys are actually filtering normally. People with low muscle mass (older adults, those with cachexia) have the opposite problem: their eGFR is overestimated. If accuracy is critical for these patients, the 2021 CKD-EPI creatinine-cystatin C combined equation is preferred, because cystatin C is not produced by muscle and is therefore unaffected by body composition.
Sources
- Inker LA, et al. New Creatinine- and Cystatin C-Based Equations to Estimate GFR without Race. N Engl J Med. 2021;385(19):1737-1749.
- KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International Supplements. 2013;3(1):1-150.
- National Kidney Foundation. CKD-EPI Creatinine Equation (2021).